The present invention relates generally to gas turbine engines and more particularly to a method of manufacturing a recuperator for a gas turbine engine.
Gas turbine engine recuperators transfer heat from the relatively hot, low pressure engine exhaust gas to incoming, relatively cold, high pressure combustion air. Use of a recuperator enables the gas turbine engine to approach the fuel economy of a diesel engine as well as to exhibit emission levels below ULEV standards. Moreover, relatively low emission levels can be achieved by recuperator equipped gas turbine engines without the use of a catalytic converter.
However, due to manufacturing complexity, known recuperators, particularly metallic plate-and-fin recuperators of the type disclosed in U.S. Pat. No. 3,507,115, are one of the most expensive components of a gas turbine engine. Moreover, known gas turbine engine recuperators, generally employ conventional seals between the hot and cold sections of the recuperator which rapidly degrade in the gas turbine engine environment.
The aforesaid problems are solved by the manufacturing method of the present invention. Specifically, the recuperator comprises an annular matrix of cells each of which comprises a low and a high pressure plate, Each plate is made from stainless steel foil that is embossed with an array of undulating ribs. The ribs stop short of the plate edges to facilitate welding to one another along the periphery thereof. The undulating ribs on the plates have different patterns to preclude nesting.
Specifically, a high pressure plate is welded to a low pressure plate by laser or resistance welding, to form a modular cell. The weld joins the flat edge portion of the low pressure plate to an offset flange on the high pressure plate that encompasses three sides thereof. The high pressure plate is provided with a short spacer bar on the radially inner edge that is also welded to the low pressure plate. A relatively longer spacer bar is welded to the exterior of the low pressure plate at the radially inner edge thereof for welding to an adjacent cell. Each cell is subsequently formed to the involute curve of the inside diameter of the annular recuperator.
A plurality of cells are assembled in an annular array to form the recuperator. After the individual cells are assembled, the protruding ends of the spacer bars on the low pressure plates define ring supports that protrude axially at each end of the recuperator annulus. Rings are then shrunk around the protruding ends of the spacer bars to fix the radially inner edges of the cells to one another but leaving radially outer portions of individual recuperator cells free to radially expand and contract.
In accordance with one feature of the invention, only the radially inner edges of the cells are brazed to one another thereby to effect joining and sealing of the cells to one another. Brazing is accomplished by rotating the assembly in a fixture at low speed while a brazing alloy, preferably a nickel or silver manganese braze alloy powder suspended in a suitable liquid carrier, is applied by a spray nozzle to the central cavity of the annular matrix of cells. The nozzle is directed radially outwardly and is moved axially of the annulus to apply a uniform coating of braze alloy to the inside diameter of the annular cell matrix.
The assembled cell matrix is then placed in either a vacuum chamber, a chamber filled with dry hydrogen, or a chamber filled with an inert gas such as argon or nitrogen. A radiation-type heating source is then inserted into the annular cell matrix which is rotated to effect uniform heating of the inner surface thereof. Distortion is minimized by heating only the inside bore of the annular cell matrix to a temperature sufficient only to melt the braze alloy and cause it to flow into the metal-to-metal contact surfaces between adjacent cells at the radially inner edges thereof, thereby allowing the bulk of the cell matrix to remain cool and unaffected by the brazing process.
After brazing, the radially inner surface of the recuperator is machined to define seating surfaces to facilitate assembly of the recuperator to an engine. It is to be noted that the radially outer surfaces of the recuperator cells are not brazed to one another, allowing relative motion between cells to minimize stress caused by thermal gradients and transients.
A stainless steel cylinder is then telescoped over the annular cell matrix so as to retain the cells in the involute shape when pressurized internally.